Audio clocking in video applications

ABSTRACT

A method of operating an electronic video device such as a DVD player, wherein video clock signals and audio clock signals are derived from a system clock signal using two phase-lock loops, and these video and audio clock signals are used to process encoded video data and encoded audio data, but digital-to-analog conversion of the audio data stream is controlled by the system clock signal rather than the audio clock signals. By using the system clock signal to control the audio digital-to-analog converter (DAC), the DAC avoids the poor performance issues that can arise from jitter introduced into the audio clock signals by the PLL. The system clock signal may be divided by an integer to generate the sampling clock for the audio DAC. In the illustrative embodiment, the system clock signal has a rate which is not an integer multiple of the sample rate of the audio data stream.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/856,436 filed May 28, 2004 now U.S. Pat. No. 7,587,131.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to clocking systems forelectronic circuits, and more particularly to a method of generatingclocking signals for audio components in an electronic video device.

2. Description of the Related Art

Various types of electronic circuits have been constructed that supporta wide range of different clock modes. For example, digital dataconverters used in audio and video devices (players or recorders) canoperate in different speed modes wherein different master clock ratesand sample clock rates are used. A digital-to-analog converter in such adevice might have two operating modes, such as a base mode and a highmode, depending on what master clock rate and sample clock rate arebeing provided from the front-end circuitry. This capability allows asingle converter to support multiple applications, and gives theend-user (i.e., the final product manufacturer) greater flexibility inthe design of the overall electronic system.

A typical clocking system for a conventional digital versatile disc(DVD) player 10 is illustrated in FIG. 1. A piezoelectric crystal 12 isused by an oscillator 14 to create a master clock signal which is fed totwo phase-lock loops (PLLs) 16 and 18. In a typical video system,oscillator 14 provides a 27 megahertz (MHz) signal which facilitatesclock generation for both U.S. and international video standards. AudioPLL 16 provides an audio clock signal for an audio processing unit 20,and optional video PLL 18 provides a video clock signal for a videoprocessing unit 22. For high performance video systems, the video PLLmay be used to multiply the clock to a higher frequency, such as 54 MHzfor 108 MHz. An optical reader 24 reads the information opticallyrecorded on the DVD and provides data signals to audio processing unit20 and video processing unit 22. Audio processing unit 20 and videoprocessing unit 22 provide various digital signal processing (DSP)functions, such as decompression of encoded audio and video datastreams. The decompressed digital audio data stream is fed to a firstdigital-to-analog controller 26, and the decompressed digital video datastream is fed to a second digital-to-analog controller (DAC) 28. DAC 26produces the audio output of DVD player 10, and DAC 28 produces thevideo output of DVD player 10. These outputs drive the audio and videooutput ports of DVD player 10 which allow interconnection to audio andvideo devices, i.e., speakers and a display.

DAC 26 is controlled by clock signals provided from audio PLL 16, andDAC 28 is controlled by clock signals provided from video PLL 18. Forexample, DAC 26 receives a master clock signal and a sample rate clocksignal which are used to sample the digital audio data from audioprocessing unit 20 and generate the proper analog audio output. Themaster clock signal might be in the range of 8 MHz to 34 MHz, and thesample rate clock signal might be in the range of 32 kilohertz (kHz) to192 kHz. The master clock is normally a multiple (such as 256) of thebase sampling rate. In a typical digital-to-analog converter, adelta-sigma modulator feeds a multilevel noise-shaped signal based onthe digital input stream to a back-end analog filter which removes highfrequencies from the output. The clock signals from audio PLL 16 areused by internal components of DAC 26 such as the delta-sigma modulatorand the analog filter. As the audio master clock, e.g. 44100*256 Hz, isnot a simple rational multiplier of the master crystal oscillator, thequality of the clock typically suffers due to a low frequency of thelock signal and typical PLL noise issues. While it is possible to buildclock generation systems that avoid this noise, most video systems haveaudio clocks with significant jitter and phase noise.

Data converters can be quite sensitive to clock noise and jitter. Jittercan move the effective sampling time of the signal, causing modulationsidebands and distortion. This situation is especially true forhigh-frequency, high-level signals. Additionally, the out-of-bandmodulation noise in delta-sigma structures can be de-modulated to becomein-band noise signals. These effects degrade both dynamic range, andsignal-to-noise ratio (SNR) measurements.

One problem that can arise in providing these critical timing signals toDAC 26 relates to the jitter associated with audio PLL 16. A phase-lockloop is a feedback device which includes a phase/frequency detector, alow-pass filter, and a voltage-controlled oscillator (VCO). Thephase/frequency detector compares two input signals, a reference signal(from the external system clock, i.e., oscillator 14) and a feedbacksignal, and generates a phase error signal that is a measure of theirphase difference. The phase error signal from the detector is filteredby the low-pass filter and fed into the control input of the VCO. TheVCO generates a periodic signal with a frequency which is controlled bythe filtered phase error signal. The VCO output is coupled to thefeedback input of the phase/frequency detector, thereby forming afeedback loop. If the frequency of the feedback signal is not equal tothe frequency of the reference signal, the filtered phase error signalcauses the VCO frequency to shift toward the frequency of the referencesignal, until the VCO finally locks onto the frequency of the reference.The output of the VCO is then used as the synchronized signal.

Jitter can be introduced into the feedback loop due to the “dead zone.”The phase error signal that controls the VCO has a first polarity in thecase where the reference signal has a phase lag, and the other polaritywhen a phase lead is detected. For very small phase differences (e.g.,the zero-phase error, steady-state condition of the locked PLL), in thetransition from one polarity to the other there is often a regionreferred to as the dead zone where the phase error signal is insensitiveto phase-difference changes. In this dead zone (or dead band), the VCO'seventual output signal is unpredictable and liable to dither.Additionally, noise may be injected into the VCO from other circuitrythat is above the corner frequency of the PLL feedback loop.

In light of the foregoing, it would be desirable to devise an improvedmethod of providing clock signals to audio components of an electronicvideo device which could avoid the performance problems associated withPLL jitter. It would be further advantageous if the method could beimplemented without significantly increasing hardware size.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide animproved method and system of clocking audio components in an electronicvideo device such as a DVD player.

It is another object of the present invention to provide such a methodand system which allow certain audio components that are more sensitiveto clock quality to be driven by a more stable clock signal.

It is yet another object of the present invention to provide an improvedclocking system for a digital-to-analog converter used in audio signalprocessing.

The foregoing objects are achieved in a method of operating anelectronic video device, generally comprising the steps of generating asystem clock signal, deriving video clock signals from the system clocksignal using a first phase-lock loop and deriving audio clock signalsfrom the system clock signal using a second phase-lock loop, processingencoded video data using the video clock signals to generate a videodata stream and processing encoded audio data using the audio clocksignals to generate an audio data stream, converting the video datastream into video output using the clock signals, and converting theaudio data stream into audio output using the system clock signal. Byusing the system clock signal to control the audio digital-to-analogconverter (DAC) rather than the audio clock signals, the DAC avoids thepoor performance issues that can arise from the jitter introduced intothe audio clock signals by the PLL. The system clock signal may bedivided by an integer to generate the sampling clock for the audio DAC.In the illustrative embodiment, the system clock signal has a rate whichis not an integer multiple of the sample rate of the audio data stream.For example, the system clock rate might be 27 MHz while the sample rateof the audio data stream is 44.1 kHz. More generally, the system clocksignal preferably has a rate of 108IN MHz, where N is an integer. An Nof 16 would give an audio converter rate of 6.75 MHz, which is anappropriate frequency for a delta-sigma audio converter. A sample rateconversion (SRC) unit in the audio DAC is used to convert the samplerate of the audio data stream to the rate of the system clock signal.The SRC unit feeds the converted stream to a delta-sigma modulator whichproduces a multilevel noise-shaped signal based on the output digitaldata stream, and an analog filter passes the multilevel noise-shapedsignal to the audio output port. The invention may be implemented in avariety of electronic video players such as a DVD player.

The above as well as additional objectives, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a block diagram of a conventional digital video disc (DVD)player having video and audio processing units which are controlled byclocking signals from respective video and audio phase-lock loops(PLLs);

FIG. 2 is a block diagram of one embodiment of a DVD player constructedin accordance with the present invention wherein a portion of the audioprocessing is controlled by clock signals from an audio PLL, while thedigital-to-analog converter (DAC) for the audio processing is controlledby a separate clock signal derived from the system clock; and

FIG. 3 is a block diagram of one embodiment of the DAC used in the DVDplayer of FIG. 2 and constructed in accordance with the presentinvention, illustrating clock control of the delta-sigma modulator andanalog filter.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention is directed to a method and system for providingclock signals to audio components in an electronic video device, and isuseful in a variety of video applications. For purposes of illustration,the invention is described below in the context of a digital video disc(DVD) player, but those skilled in the art will appreciate that theinvention is not limited to this application and can be implemented inother types of video players as well as video recorders.

With reference now to FIG. 2, there is depicted one embodiment of a DVDplayer 30 constructed in accordance with the present invention. DVDplayer 30 includes an optical reader 32 which reads thedigitally-encoded audio and video information optically recorded on theDVD. Optical reader 32 provides separate outputs for the encoded(compressed) audio and video data. The audio data is sent to an audioprocessing unit 34, and the video data is sent to a video processingunit 36. Audio processing unit 34 and video processing unit 36 mayprovide various digital signal 15 processing (DSP) functions includingdecompression of the encoded audio and video data streams. Audioprocessing unit 34 and video processing unit 36 are controlled by clocksignals generated at respective audio and video phase-lock loops (PLLs)38 and 40. The audio and video PLL clock signals are derived from asystem clock signal produced by oscillator 42 using a piezoelectriccrystal 44. The output of audio processing unit 34 is 20 connected to anaudio digital-to-analog converter (DAC) 46, and the output of videoprocessing unit 36 is connected to a video DAC 48. The output of audioDAC 46 is the audio output for DVD player 30, and the output of videoDAC 48 is the video output for DVD player 30.

While the present invention provides an advantageous and novel clockingsystem for DVD player 30, many of the individual components of player 30are of conventional design (e.g., optical reader 32, oscillator 42,audio and video PLLs 38 and 40, audio and video processing units 34 and36, and video DAC 48). The details of these components are accordinglybeyond the scope of the present invention but will become apparent tothe system designer upon reference to this disclosure.

In the illustrative embodiment, exemplary oscillator 42 derives thesystem clock from a 27 MHz crystal. A system clock of 27 MHz easilydrives both National Telecommunications Standards Committee (NTSC) andProgressive Alternating Line (PAL) devices. This value is exemplary andmay vary depending upon the application. Integer multiples of 27 MHz(e.g., 54 MHz) are suitable as well.

The audio and video PLL clocks are derived from the 27 MHz system clock.Audio PLL 38 produces a master clock signal and a sample rate clocksignal for audio processing. The master clock signal is a multiple (suchas 256) of the base audio sampling rate. In this embodiment, the masterclock is 11.2896 MHz, and the sample rate clock 44.1 kHz. Video PLL 40also produces a master clock signal for video processing, for example,54 MHz. These values are exemplary and other rates could be used, e.g.,a 48 kHz sample rate clock.

Some components such as DACs can be more sensitive to clock quality andare particularly subject to poor performance associated with PLL jitter.The present invention avoids this problem by using the system clock (27MHz) provided by oscillator 42 for audio conversion, and sample rateconverting the audio data stream using a clock rate of 27 MHz/N where Nis an integer. In this implementation, the signal from oscillator 42 isinput to a divider 50 whose output provides the clock signal to DAC 46.For example, divider 50 can divide the signal by 4 to present a 6.75 MHzclock to the audio DAC. DAC 46 can use clock signals derived from the 27MHz system clock, even though it is not an integer multiple of thesample rate for the audio data stream (44.1 kHz), by using sample rateconversion. More generally, it is preferable to operate DAC 46 at a rateof 108/N MHz, where N is an integer.

FIG. 3 illustrates one embodiment of DAC 46. The 44.1 kHz data stream isfed into a sample rate conversion (SRC) unit 52. SRC unit 52 has theratio of the base audio sample rate (44.1 kHz) to the system-derivedsample rate (13.5 MHz) and uses interpolation to convert the datastream. Inclusion of SRC unit 52 in the design of DAC 46 does notsignificantly increase the overall hardware size of the device. SRC 52can be designed to be simpler than general-case SRC units, as the outputsampling rate is always quite high with respect to the signal bandwidth.The output of SRC unit 52 is provided to a delta-sigma modulator 54.Delta-sigma modulator 54 feeds a multilevel noise-shaped signal based onthe digital input stream to an analog filter 56. Delta-sigma modulator54 can be operated at 6.75 MHz for switched capacitor systems, ordirectly at 27 MHz for continuous time systems. Analog filter 56 ispreferably a continuous-time filter. Analog filter 56 removes highfrequencies from the output, and the filtered output then drives someother device such as an amplifier 58 and speaker 60. Amplifier 58 andspeaker 60 may be internal or external to DVD player 30.

As the audio clock is no longer used to clock any critical converterresources, the audio clock no longer needs to be a smooth, continuousclock; rather, the audio clock only needs to have the right averagefrequency such that the appropriate amount of audio data is processed ina given macroscopic time. The system may be designed without any PLLarid true audio clock, allowing a bursty transfer clock for audioderived directly from the system clock.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore contemplated that such modifications can bemade without departing from the spirit or scope of the present inventionas defined in the appended claims.

1. A method of operating an audio converter, comprising: deriving anaudio converter clock signal; processing encoded audio data to generatean audio data stream sampled at an audio sample rate; and converting theaudio data stream into audio output using the audio converter clocksignal, wherein the frequency of the audio converter clock signal is anon-integer multiple of the audio sample rate, and the audio converterclock signal has a frequency equal to 27 MHz multiplied by N/M, where Nand M are integers and M is less than or equal to
 16. 2. Adigital-to-analog converter comprising: a system clock input forreceiving a system clock signal; a sample rate conversion unit whichconverts an input digital data stream having a first audio sampling rateinto an output digital data stream having a second audio sampling rateequal to a rate of the system clock signal divided by an integer; adelta-sigma modulator which produces a multilevel noise-shaped signalbased on the output digital data stream; and an analog filter whichreceives the multilevel noise-shaped signal.
 3. The digital-to-analogconverter of claim 2 wherein said delta-sigma modulator is aswitch-capacitor modulator that operates at a rate equal to the rate ofthe system clock signal divided by the integer.
 4. The digital-to-analogconverter of claim 2 wherein said delta-sigma modulator is acontinuous-time modulator that operates directly at the rate of thesystem clock signal.
 5. The digital-to-analog converter of claim 2wherein: the system clock signal has a rate of 27 MHz; and the inputdigital data stream has a first audio sampling rate selected from thegroup consisting of 44.1 kHz or 48 kHz.
 6. The digital-to-analogconverter of claim 2 wherein the system clock signal has a rate of 108/NMHz, where N is an integer.
 7. The digital-to-analog converter of claim2 wherein the system clock signal has a rate which is not an integermultiple of a rate of the first audio sampling rate.
 8. Thedigital-to-analog converter of claim 2 wherein said analog filter is acontinuous-time analog filter.
 9. A method of operating an audioconverter, comprising: deriving an audio converter clock signal;processing encoded audio data to generate an audio data stream sampledat an audio sample rate; and converting the audio data stream into audiooutput using the audio converter clock signal, wherein the frequency ofthe audio converter clock signal is a non-integer multiple of the audiosample rate, and the audio converter clock signal has a frequency equalto 54 MHz multiplied by N/M, where N and M are integers and M is lessthan or equal to
 32. 10. A method of operating an audio converter,comprising: deriving an audio converter clock signal; processing encodedaudio data to generate an audio data stream sampled at an audio samplerate; and converting the audio data stream into audio output using theaudio converter clock signal, wherein the frequency of the audioconverter clock signal is a non-integer multiple of the audio samplerate, and the audio converter clock signal has a frequency equal to 108MHz multiplied by N/M, where N and M are integers and M is less than orequal to 64.